Abstract

One of the grand challenges of contemporary physics is understanding strongly interacting quantum systems comprising such diverse examples as ultracold atoms in traps, electrons in high-temperature superconductors and nuclear matter(1). Warm dense matter, defined by temperatures of a few electron volts and densities comparable with solids, is a complex state of such interacting matter(2). Moreover, the study of warm dense matter states has practical applications for controlled thermonuclear fusion, where it is encountered during the implosion phase(3), and it also represents laboratory analogues of astrophysical environments found in the core of planets and the crusts of old stars(4,5). Here we demonstrate how warm dense matter states can be diagnosed and structural properties can be obtained by inelastic X-ray scattering measurements on a compressed lithium sample. Combining experiments and ab initio simulations enables us to determine its microscopic state and to evaluate more approximate theoretical models for the ionic structure.